Television transmitting and the like system



April 4, 1939. I w. F. TEDHAM I 2,153,163

TELEVISION TRANSMITTING AND THE LIKE SYSTEM Filed Nov. 25, 1954 25Man-sacs Patented Apr. 4, 1939 UNITED STATES PATENT OFFICE TELEVISIONTRANSMITTING AND THE LIKE SYSTEM Application November 23, 1934, SerialNo. 754,385 In Great Britain December 6, 1933 6 Claims.

A television transmitter is known in which an optical image of theobject to be transmitted is projected upon a mosaic screen ofphoto-electrically active, insulated elements and the screen scanned bya cathode ray. Each time an element is scanned its potential 1s loweredto a fixed datum level by the electrons of the cathode ray and inbetween scans is raised, by photo emission of electrons, to a valuedetermined by the intensity of light falling upon the mosaic screen fromthe object. Now it has been found that if, in such a tube, an oxidecoated cathode is used, as is desirable since these cathodes have a highemission, a small quantity of gas which is probably oxygen is emittedslowly from the cathode; whilst this gas is insuificient to soften thetube appreciably it has the effect of damaging the photo-electricelements of the mosaic screen, when these are of caesium on silver forexample, by oxidising the free caesium.

A transmitter is also known in which an optical image of the object tobe transmitted is projected intermittently upon a mosaic photoelectricscreen and the screen scanned by a light beam during those periods whenthe screen is unilluminated by light from the object. In between eachscan each element acquires a charge proportional to the time for whichlight from the object falls upon it and since light does not fallcontinuously upon the object this charge is comparatively small andseriously reduces the efficiency of the system. Also, since the lightbeam is operated by mechanical means the arrangement suffers from wellknown disadvantages inherent in mechanically operated scanning systemssuch, for example, as those arising out of the large inertia of themoving parts.

It is an object of the present invention to provide a televisiontransmitter in which a mosaic screen of photo-electrically activeelements is scanned with the aid of a radiant energy, such as light,which is comparatively harmless to the elements.

It is another object of the present invention to provide a method oftransmitting images of an object to a distance in which an optical imageof the object to be transmitted is projected continuously upon a mosaicscreen of photo-electrically active elements whilst the elements arescanned by a beam of radiant energy, the optical image operating duringsubstantially the whole time between successive scans of any oneelement, to produce a photo-electric emission of electrons from thatelement.

The invention will now be described with the aid of the accompanyingdiagrammatic drawing, in which Fig. 1 illustrates televisiontransmitting apparatus arranged and adapted to operate in accordancewith the present invention, and

Fig. 2 is a graph illustrating the functioning of the apparatus of Fig.1.

Referring now to Fig. 1, a television transmitter comprises a cathoderay tube l and a separate evacuated glass cell 2 containing twoelectrodes. 10 One of the electrodes of the cell, namely the oathode 3,consists of an aluminium plate 4 bearing on one face a thin uniformlayer of aluminium oxide 5 which in turn bears a mosaic screen 6 ofphoto-electrically active elements separated 15 from one another.Henceforth the aluminium plate 4 will be calledthe signal plate of thetube. The thickness of the oxide layer 5 is such that if an element ofscreen 6 is maintained at a few volts positive with respect to thesignal plate 20 4 a leakage of electrons occurs from the plate to theelement, so that although the elements will be spoken of as beinginsulated from one another, the insulation is not perfect. There may beten thousand elements to a square inch and each ele- 25 ment mayconsist, for example, of caesium deposited upon silver oxide. The anode1 of the cell is in the form of a plane sheet of fine, widemesh, wiregauze and is disposed a short distance away from the cathode 3 in aplane parallel to 30 the latter electrode. The anode i is maintained at,for example, twenty volts positive with respect to the signal plate 4 ofthe cathode 3 and both electrodes are disposed in planes parallel to aplane, transparent, end wall 8 of the cell 2. 35

Facing this end wall 8 is the cathode ray tube l which may be of anyknown or suitable kind. The tube is provided with a fluorescent screen9, (preferably of a material such as calcium tungstate, which has ashort time lag) disposed in a 40 plane parallel to the wall 8, and withmeans such as two pairs of electrostatic plates l0 and II, for causingthe ray to scan the fluorescent screen 9 in any suitable manner,preferably at constant speed. 45

Between the tube l and the cell 2 is an optical system l2 adapted toproject an image of the moving fluorescent spot through the wire meshanode 1 of the cell 2 on to the mosaic screen 6, the mosaic screen beingscanned in this manner 50 with a light beam of substantially constantintensity derived from the cathode ray tube i.

Also disposed between the cell i and the tube 2 is a half-silveredmirror l3, inclined at about 45 to the planes of the cell electrodes 3and 1. This 55 mirror is adapted, in conjunction with suitable lenses M,to project an image of the object IE (to be transmitted) through theanode l of the cell 2 on to the mosaic screen 6. The image of the objecti is projected continuously onto the mosaic screen 6 and scanning ofthis screen, which is eifected simultaneously, is also carried out ascontinuously as possible. If interruptions in the scanning process aremade, say for example, to enable synchronising signals to be interposedbetween trains of picture signals, these interruptions should be made asshort as possible.

If desired the half-silvered mirror 83 may be replaced by an aperturedmirror, the scanning beam of light passing from the tube I to the cell 2through the aperture in the mirror.

The anode 'l of the cell 2 is connected to the positive terminal of anelectric current source it, the other terminal of the source beingconnected to one end of a high load resistance Ill and to earth. Theother end of the load resistance i1 is connected to the signal plate 4and potentials developed across the load resistance H are amplified inan amplifier I8, superimposed upon a carrier wave, generated in anoscillator l9, by means of a modulator and transmitted from aerial 2!.

The transmitter is operated as follows:

An optical image of the object E5 to be transmitted, whether it bemoving or stationary, is projected upon the photo-electric surface ofthe mosaic screen 6 and the latter is simultaneously scanned with lightderived from the cathode ray tube l, at a rate of twenty-five times persecond, for example.

The potential changes of an element during scanning, relative to theanode potential, are illustrated in Fig. 2, in which ordinates (VE)represent the potential of an element and abscissae represent time. Thepotential of the anode 7 remains constant at the value shown by line AB.

The intensity of the scanning light beam, which is equivalent to aboutone lumen per square inch, is sufficient to cause a saturation emissionof electrons from the photo-electrically active element scanned to thewire gauze anode 1. Consequently each time an element is scanned it isquickly brought up to the potential of the anode, that is to say, israised to twenty volts positive with respect to the potential of thesignal plate 4. If twenty-five complete scans of the object take placeevery second, the potential of each element is brought to the level AB(Fig. 2) every th of a second. At each scan the leakage of electronsacross the oxide layer 5 is negligible compared with the photo-electricsaturation emission of electrons.

After the scanning beam passes ofi an element, the photo-electricemission drops to a value which is below the saturation value 'and whichis dependent upon the intensity of the light reaching the element fromthe object. During this time, that is to say, in between successivescans, the leakage of electrons across the oxide layer 5 is comparablewith, but never less than, the photoelectric emission of electrons.Thus, if no light is reaching an element from the object, immediatelyafter the scanning beam passes off the element, the photo-electricemission drops to zero and the potential of the element falls to that ofthe signal plate t (shown at CD in Fig. 2) owing to the leakage ofelectrons across the oxide layer, On the other hand if much light fallsupon an element from the object, immediately after the scanning beampasses off the element, the photoelectric emission falls to a steadyvalue which is below the saturation value and which is determined by theintensity of the light reaching it. At this stage, however, the leakageof electrons from the signal plate 4 to the element is greater than thephoto-electric-emission of electrons from the element to the anode 'I.The potential of the element relative to the plate 4 therefore falls andthis in turn reduces the leakage from the signal plate 4, an equilibriumstate is in this manner quickly arrived at when the leakage is equal tothe photo-electric emission and at this stage the potential of theelement remains steady, at a value (such as that indicated by line EF ofFig. 2) between that of the signal plate 4 and the anode l, until it isonce again brought up to the datum value (that is to say, to thepotential of the anode 1) by the scanning beam.

As the scanning beam passes over the whole mossaic screen 6, thepotential of each element is in turn raised, relative to that of thesignal plate 4, from a value dependent upon the intensity of the lightreaching the element from the object to its datum value (represented byline AB in Fig. 2) and then allowed to fall back to a value determinedby the nature of the object.

The potential pulses generated in this manner are transmitted throughthe small condensers formed by the individual mosaic elements and thesignal plate 4, the oxide layer 5 constituting the dielectric anddevelop corresponding potential differences across the resistance l'l.

Now a mosaic screen having 10,000 elements to a square inch can beconstructed conveniently and the thickness of the oxide layer 5 can bemade such that each element has a capacity of 5 10 farads to the signalplate 5. A good photo-electric surface will emit 14.4L 10 amperes perlumen and the maximum illumination of the mosaic screen 6 which can bederived from a bright object is about 10 foot-candles.

An illumination of 10 foot-candles is produced by 10 lumens falling uponan area of 1 square foot. Therefore an illumination of 10 footcandles onone square foot of photo-electric surface produces a photo-electricemission of 10 (14.4 1D*) amperes, so that this illumination causes oneelement of area 10- sq. inch to emit amperes or 10 amperes.

Assuming for the moment that the oxide layer 5 acts as a perfectinsulator, the charge acquired by an element in between successivescans, that is to say, in second, is 10- coulombs, and

the potential acquired by the element in this time is therefore 5 X 10-volts or 80 volts.

In practice, however, the oxide layer 5 is not a perfect insulator, themaximum illumination to be expected from the object may be less than 10foot-candles and the photo-electric emission from a mosaic surface maybe less than 1e.4 0

volts so that the above condition is satisfied if the resistance of theoxide layer between one element (of area sq. inch) and the plate is 10ohms. That is to say, with the values assumed above, the oxide layer 5must have a resistance of 10 megohms per square inch. If it is less thanthis the picture signals will be of reduced intensity. The value of theresistance is adjusted by controlling the oxidation of the aluminiumsignal plate 4.

With the figures given above it is found that each element is raised toits datum potential practically instantaneously when scanned and fallsto its equilibrium or picture potential in about 25 milliseconds.

The layer 5 of aluminium oxide between the aluminium signal plate andthe photo-electric elements may be replaced by a thin layer of avitreous enamel. Alternatively the mosaic screen 6 of mutually insulatedphoto-electric elements may be replaced by a highly-resistive continuousscreen of photo-electric material provided that the screen has asufficiently high transverse resistance to prevent the charges acquiredby small elements of the screen from leaking away to neighbouringelements at an appreciable rate.

In the pref-erred form of the invention scanning is effectedwith the aidof a light beam derived from a cathode ray tube, but it lies within thescope of the invention to use for scanning a light beam which ismechanically swept over the mosaic screen. In this case, however, thedifiiculties inherent in mechanically operated scanning apparatus areintroduced.

I claim:

1. A television transmitting apparatus comprising a sealed envelopecontaining a signal electrode, a screen consisting of a plurality ofphoto-electrically active elements, an anode, and a partial insulationbetween said elements themselves and them and said signal electrode;means for maintaining said anode at a positive potential with respect tosaid signal electrode; optical means for projecting an image of theobject to be transmitted upon said elements causing them to emitphoto-electrons; means for scanning said elements with a beam of radiantenergy such as light causing another emission of photo-electrons, saidemissions resulting in leakage currents from any of said elementsthrough said partial insulation to said signal electrode, said leakagecurrents resulting from scanning being at least as great as any of saidother leakage currents; an output circuit associated with said signalelectrode; and means for transmitting signals generated in said outputcircuit.

2. A television transmitting apparatus comprising a sealed envelopecontaining a signal electrode, a screen consisting of a plurality ofphotoelectrically active elements, an anode, and a partial insulationbetween said elements themselves and them and said signal electrode;means for maintaining said anode at a positive potential with respect.to said signal electrode; optical means for continuously projecting animage of the object to be transmitted upon said elements causing them toemit photo-electrons; means for scanning said elements with a beam ofradiant energy such as light causing another emission ofphoto-electrons, said emissions resulting in leakage currents from anyof said elements through said partial insulation to said signalelectrode, said leakage currents resulting from scanning being at leastas great as any of said other leakage currents; an output circuitassociated with said signal electrode; and means for transmittingsignals generated in said output circuit.

3. A television transmitting apparatus comprising a sealed envelopecontaining a signal electrode, a screen consisting of a plurality ofphoto-electrically active elements, an anode and a partial insulationbetween said elements themselves and them and said signal electrode;means for maintaining said anode at a positive potential with respect tosaid signal electrode; optical means for continuously projecting animage of the object to be transmitted upon said elements causing them toemit photo-electrons; means for scanning said elements with a beam ofradiant energy such as light causing another emission ofphotc-electrons, said partial insulation being so as to permit leakagecurrents resulting from said emissions to flow from any of said elementsto said signal electrode, whereby said leakage currents resulting fromscanning are at least as great as any of said other leakage currentsresulting from illumination by said object so that the potential of anelement not being illuminated by said object falls down substantially tothat of the signal electrode within the interval between successivescans thereof; an output circuit associated with said signal electrode;and means for transmitting signals generated in said output circuit.

4. Television transmitting apparatus including a sealed envelope havingdisposed within it a mosaic screen of mutually insulatedphoto-electrically active elements, a signal electrode partiallyinsulated from said elements and an anode, means for maintaining saidanode at a positive potential with respect to said signal electrode, anoptical system for projecting an optical image of the object to betransmitted continuously upon said elements, a cathode ray tube having afluorescent screen, deflecting means for scanning said fluorescentscreen with the cathode ray, means for scanning said elements with lightderived from said fluorescent screen, an output circuit associated withsaid signal electrode and means for transmitting signals generated insaid output circuit.

5. In a method of television transmission the steps of projecting anoptical image of the object to be transmitted upon a screen comprisingmutually insulated photo-electrically active elements, scanning saidscreen to raise elements thereof successively to a fixed datum level ofpotential and in the intervals between successive scans of an element,reducing the potential of said element, relative to said datum level, toa value dependent upon the brightness of the light with which saidelement is illuminated by said image in said intervals, and utilizingthe changes of potential of said elements to provide signals which areamplified and transmitted.

6. In a method of television transmission the steps of projecting anoptical image of the object to be transmitted upon a photo-electricallyactive screen, scanning said screen to bring elements thereofsuccessiveiy to a fixed datum level of potential and in the intervalsbetween successive scans of an element, lowering the potential of saidelement, relative to said datum level, to a value determined by thedifierence between the photo-electric emission of electrons from saidelement and a leakage of electrons tosaid element, and utilizing thechanges of potential of said elements to provide signals which areamplified and transmitted.

WILLIAM FRANCIS TEDHAM.

